The present invention relates generally to transponder communications systems, and more particularly, to a method that controls the systematic exchange of data between a transponder in a vehicle and a reader by processing data transmissions in a plurality of detection regions that cover less than the entire tracking zone to minimize the amount of data transferred therebetween.
The assignee of the present invention has developed an open road toll road revenue collection system that computes tolls for vehicles using a toll road. A vehicle-to-roadside communications (VRC) system is used to exchange data between transponders in vehicles and a reader. The VRC system is an over-the-air, line-of-sight two way communication system. The VRC system transfers data from a memory of the transponder to a roadside reader and from the reader to the memory of the transponder. The reader continuously outputs frame messages in a predetermined radio frequency (RF) band.
Toll collection sites are set up at entry and exit ramps of the toll road. When the transponder moves within a communications zone of a toll collection site, it detects the transmitted reader frame messages, wakes up and attempts to decode the message. When the reader frame message is decoded correctly, the transponder is connected to the system, and transmits its transponder ID code. The reader then assigns a time slot in a message frame in which the transponder transmits its memory contents.
A transponder locator listens to radio frequency transmissions from the transponder. The transponder locator uses multiple antennas with phase array elements to determine the angle of arrival of the transmitted RF signals at each antenna. These angle of arrival measurements are combined and the geolocation of the transponder is determined. Measurements made at different times and at multiple transponder locations are processed to determine a track on the road of the path of the transponder.
The communication zone in which transponders can communicate with the reader cannot be configured such that the reader only communicates with transponders in a certain area. The communication zone is larger than the toll collection site. Consequently, the reader has the ability to communicate with transponders that are not exiting or entering the toll road. This increases the processing required by both the reader and the locator.
Prior to the present invention, the transponder locator generated position data every time a transponder ID code was correctly received while the transponder was in the communication zone. The raw data was then processed by a processor to make a positional graph. Thus, the locating scheme used prior to the present invention had the transponder locator continuously report the position of a transponder to the processor and display system when the transponder was in the communication zone.
The disadvantage of this approach was that the transponder locator sent data to the processor every 10 milliseconds to update the position of the transponder. Prior to the present invention, the transponder locator could have been required to send over 200 messages for the same vehicle while it was passing through the communications zone. The reader communicates with multiple transponders, and the transponder locator tracks multiple transponders. In a situation with many transponders communicating with the transponder locator, the transponder locator was required to output more than 30,000 bits of information per second. It was determined that a reduction in the amount of data transferred by the transponder locator without impairing system performance would be a benefit.
Thus, it is an objective of the present invention to provide for a method that minimizes data transfer between the transponder locator and processor. It is a further objective of the present to provide a method wherein detection regions are used to determine entry into the toll zone and to determine that a transponder and its vehicle is on a different road and whose data should be discarded.